This invention relates to novel solder compositions or solder creams, and is particularly concerned with the provision of solder compositions especially adapted for soldering components during high speed fusing operations.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic balls of various alloys such as tin-lead, tin-lead-silver, tin-lead-gold, etc., contained in a vehicle including a flux such as wood rosin, or derivatives thereof. In addition, a suspension medium is present in the vehicle and is combined with the flux to provide a paste substance of uniform texture, consistency, and improved surface wetting. In addition, an organic solvent is present in the vehicle to provide the proper consistency for the vehicle.
In electronic circuitry, solder creams or compositions are employed to secure miniature electronic components to a circuit on a substrate such as ceramic or glass, or the like. Circuits similar to those used on printed circuit boards are produced on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistency is applied, for example by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board.
Many solder creams or compositions have been developed and are commercially available. However, such prior art and commercially available solder creams have the disadvantage that during soldering many small solder balls are formed around and between each of the solder pads. Many of these solder balls create electrical short circuits between adjacent pads. Further, many solder balls form beneath the components, and it is difficult, and sometimes impossible, to scrub or remove solder balls from the areas of the circuit board immediately beneath the components. Thus, due to this problem, short circuiting exists and the rejection rate for such boards has been very high.
My U.S. Pat. No. 4,373,974 discloses a novel solder composition for soldering components in electronic circuitry, whereby the formation of solder balls is essentially eliminated, thus avoiding the necessity of removing such solder balls. Such composition comprises solder metals such as tin and lead, dispersed in a vehicle containing viscosity-controlling agents, organic solvents including lower aliphatic alcohols such as butyl alcohol, and ethylene glycol, and a flux, employing a narrow range of between 13 and 14% vehicle, and between 86 and 87% of solder metal, by weight of the solder composition.
Although the composition of my above patent has proved highly satisfactory for soldering components generally in electronic circuitry during normal soldering operations, in high speed soldering or fusing operations employing such composition it has been found that movement of components occurs, causing solder ball formation.
It is accordingly an object of the present invention to provide an improved solder composition or solder cream which is especially adapted for soldering components during high speed fusing operations.
Another object is the provision of a solder composition of the above type which minimizes formation of undesirable solder balls during high speed fusing operations, and causing short circuits between electronic components.
A still further object of the invention is to provide an efficient solder composition of the general type disclosed in my above patent, and which is particularly adapted for soldering electronic components utilizing high speed soldering operations.